Allow multiple columns in the same table to be modified within the same ALTER statement (one operation)

[daniel-crlabs]

Is your feature request related to a problem? Please describe.

This is a feature request.
Currently we are not able to make multiple modifications to different columns on the same table within the same ALTER statement. We need to issue multiple ALTER statements to the same table in order to make multiple changes. This can be problematic, especially for very large tables, as the "downtime" for the table affected can be quite substantial.

The following fails when modifying 2 existing columns simultaneously within the same alter table statement on the same table:

root@lab-kub01:30007/test> alter table t3 alter column c3 type INT8, alter column c4 type INT8;
ERROR: unimplemented: ALTER COLUMN TYPE cannot be used in combination with other ALTER TABLE commands
SQLSTATE: 0A000
HINT: You have attempted to use a feature that is not yet implemented.
See: https://go.crdb.dev/issue-v/49351/v21.2

The following works, if we are adding 2 new columns:

root@lab-kub01:30007/test> alter table t3 ADD COLUMN c5 varchar, ADD COLUMN c6 int;
ALTER TABLE


Time: 568ms total (execution 568ms / network 1ms)

Describe the solution you'd like

The solution here would be to allow something other RDBMS already do (PostgreSQL does as well) which is allow someone to make multiple changes to the same table within the same ALTER statement.

i.e. This should work

alter table t3 alter column c3 type INT8, alter column c4 type INT8;

Describe alternatives you've considered

The current alternative is to issue 2 separate alter table statements as shown below:

root@lab-kub01:30007/test> alter table t3 alter column c3 type INT8;
NOTICE: ALTER COLUMN TYPE changes are finalized asynchronously; further schema changes on this table may be restricted until the job completes; some writes to the altered column may be rejected until the schema change is finalized
ALTER TABLE

Time: 1.185s total (execution 1.184s / network 0.001s)

root@lab-kub01:30007/test> alter table t3 alter column c4 type INT8;
ALTER TABLE


Time: 199ms total (execution 198ms / network 1ms)

root@lab-kub01:30007/test>

Jira issue: CRDB-15339

[ajwerner]

Relates to #49329, seems like a duplicate of #47137.